1. Field of the Invention
The present invention relates to a processing method for reducing elution of lead from a lead-containing copper alloy, a plating method for reducing elution of lead in a lead-containing copper alloy, and a drinking water service fitting made of a lead-containing copper alloy which exhibits reduced elution of lead.
The drinking water service fittings include those which are directly connected to a feed water pipe, such as water heater, water cooler, ice-maker, water purifier, hot water boiler, vending machine, pole tap, low tank, valve (faucet metals), joint, pipe, sink, wash stand, stool, bath tub, housing unit, etc.
2. Description of the Prior Art
Hitherto, faucet metals have been generally manufactured by casting or forging a copper alloy such as bronze or brass, cutting and polishing to shape, and then nickel chromium plating, etc.
To improve machinability of the copper alloy during cutting in a manufacturing process, lead is added to the copper alloy.
FIG. 7 is a schematic view of composition of the copper alloy to which lead is added. When lead is added to the copper alloy 1, lead, lead oxide, lead hydroxide or the like gather near the surface of the copper alloy, while lead 2 exists as a simple substance in the inside thereof. The concentration of the lead 2 near the surface is several times higher than that of lead on the inside.
In the faucet metals made of bronze casting to which lead is added, lead of about 500 ppb elutes. Thus, lead elutes from the surface of a water flow channel of the drinking water service fittings made of a lead-containing copper alloy into water and as a result, there is the possibility that drinking of such water for a long time will exert a bad influence on a human body.
However, copper alloy materials to which lead is not added has poor machinability, and a substitute copper alloy material has not yet been developed.
It is, therefore, an object of the present invention to provide a processing method for preventing elution of lead in a lead-containing copper alloy by removing lead from the surface of a lead-containing copper alloy material to prevent lead from eluting from a faucet metal, etc. made of a lead-containing copper alloy, a plating method for reducing elution of lead in a lead-containing copper alloy, and a drinking water service fitting made of a lead-containing copper alloy in which elution of lead has been prevented.
The present invention pays attention to a property of lead as an amphoteric metal, wherein a lead-containing copper alloy is immersed in an alkaline etching solution to which an oxidant is added, to selectively dissolve and remove lead on the surface of a lead-containing copper alloy material.
Thus, both a chemical reaction whereby lead on the surface of a lead-containing copper alloy material contacts an alkaline solution so as to directly become an alkali metal salt of a water-soluble zincate, and a chemical reaction whereby the lead is first converted to lead oxide by an oxidant and this lead oxide is converted to an alkali metal salt of a water-soluble zincate by an alkaline solution, proceed together. However, the chemical reaction of the latter advances faster than that of the former and as a result, dissolution of lead is expedited to remove lead.
When this chemical reaction occurs, copper (a simple substance), tin forming an alloy together with copper, zinc, aluminum, etc. do not react to the alkaline solution, and only the lead (a simple substance) which does not form an alloy together with copper is selectively removed by the above-mentioned chemical reaction.
It is desirable to further add a chelating agent to this etching solution. By adding the chelating agent, lead is formed with a water-soluble complex and it is possible to effectively remove lead.
Further, it is desirable to form a chromate film on the surface of a lead-containing copper alloy material after lead on the surface of the lead-containing copper alloy material is dissolved and removed.
Namely, a chemical reaction that dissolves a lead-containing copper alloy, and a chemical reaction that forms a chromate film are caused by chromic acid contained in a chromate solution so as to dissolve and remove the lead left in a limited amount on the surface of a lead-containing copper alloy material. Also, the surface of the lead-containing copper alloy material from which lead has been removed is protected by a chromate film. Lead on the inside does not elute even though the surface of the lead-containing copper alloy material from which lead has been removed corrodes due to long term water flow. It is therefore possible to reduce elution of lead for a long period.
Further, the present invention pays attention to the point that in a plating step such as nickel chromium plating to be effected on a lead-containing copper alloy, this alloy is usually immersed in a plating solution. The outer surface of the lead-containing copper alloy is therefore plated and, at the same time, lead on the inner surface is dissolved and removed.
An alkaline degreasing solution is used as a pre-cleaning step for plating, but an oxidant is contained in the solution to expedite dissolution and removal of lead on the inner surface of a lead-containing copper alloy material. Also, addition of fluoride to a chromium plating solution serves to effectively dissolve deposits of lead chromate.
Further, a lead-containing copper alloy material may be immersed in a chromate solution to form a chromate film on the inner surface thereof after plating the outer surface thereof. With this film formation, it is possible to reduce elution of lead left in a limited amount on the surface of the lead-containing copper alloy material. It is desirable to add a phosphoric acid to the chromate solution.
The lead-containing copper alloy material is degreased in an alkaline degreasing solution as a cleaning step before plating in a solution in which an oxidant is contained. Acid activation and nickel plating are subsequently effected and then, chromium plating is effected in a chromium plating solution that contains fluoride. Thus, it is possible to dissolve and remove lead on the inner surface of the lead-containing copper alloy material while effecting nickel chromium plating on the outer surface thereof.
Further, the lead-containing copper alloy material is degreased in an alkaline degreasing solution as a cleaning step before plating in a solution which contains an oxidant. Acid activation and nickel plating are subsequently effected and then, chromium plating is effected in a sargent chromium plating bath or a chromium fluoride plating bath. The lead-containing copper alloy material is further immersed in a chromate solution to form a chromate film on the inner surface thereof. It is possible to dissolve and remove lead on the inner surface of the lead-containing copper alloy material while effecting nickel chromium plating on the outer surface thereof. It is desirable to include a phosphoric acid in the chromate solution.
The faucet metals mainly use a lead-containing copper alloy material such as bronze or brass, and various plating steps are effected to improve the beauty of the outer surface, corrosion resistance, and wear and abrasion resistance. According to the present invention, plating is effected on the outer surface of the lead-containing copper alloy material while reducing elution of lead on the inner surface thereof at the same time. It is therefore possible to reduce elution of lead in various plating steps including nickel chromium plating.